Hello,

I did a migration from elasticsearch 2.3.4 to 5.1.1 by following the migration guide.
The migration went perfectly well and I updated my mapping in order to use the new keyword type instead of the string not analyzed old one.

So I wanted to reindex all my index and I encountered the 2 following problems :

• some of my documents had id of length longer than 512 characters and elasticsearch 5.1.1 does not accept it while a reindexation.
• some of my documents had fields whose name is an empty string : elasticsearch 5.1.1 refuses to reindex such field.

So my reindex task suddenly stops, leaving lots of my documents not being reindexed because of the error on one document.

The improvement I recommend is to introduce some robustness of the reindex processing that in case of failure of some documents, continues normally with all others documents present in the index.

Linked to this behavior, it would be great to add in the reindex API, the possibility to get the result message of the reindex task once finished (including the number of successes and failures). Indeed, when run through kibana devmode, the json response is not displayed because of client timeout for large index.

In my case, i was not able to correct the data of my old indexes so I decided not to reindex them finally...

Regards,